Aerobic composting of organic residual material such as manure, garbage, sewage sludge, mixed solid waste, leaves, and green wastes, yields a product which is a valuable soil conditioner. The composting process requires a continuous supply of oxygen to the microbial population in the compost feed material to decompose the organic portion of the material into a stable, usable form. Old composting processes utilized open field windrows and aerated static piles to achieve this. To enhance these processes, machines for mixing the windrows, such as the SCARAB .TM.machine by the Scarab Manufacturing Company, were used. Bulking agents, such as sawdust, were also used by mixing them with the feed material permitting greater air contact with the feed material.
A relatively new development in composting technology is the use of in-vessel composters whereby the composting process takes place in a sealed container. In order to obtain an acceptable product, a forced aeration system must be utilized to push air through the compost feed material. The volume of air used must be carefully regulated in order to supply sufficient amounts of oxygen to the microbial population, to evaporate moisture from the compost mass so as to obtain a final product having approximately 40-45% moisture by weight, and to cool the composting temperature should it exceed about 160.degree. F. (inhibiting bacterial action). In-vessel composting has a number of advantages over windrow composting including, a shorter composting cycle, independence from weather problems and a significant reduction in dust and odor.
Many existing composting installations use a combination of in-vessel composting systems and some of the older prior art techniques. Table 1 describes the composting systems used by major composting facilities in the United States.
A significant problem of composting in general is that the chemical and biological reactions occurring between the compost feed material, the microbial population, and the aeration air create a wide assortment of odorous compounds. Such compounds include ammonia, organic sulfides and aldehydes. In addition, toxic volatile organic compounds are generated by the self-heating biological process which volatilizes these compounds from the feed material. For example, volatile compounds in sewage sludge includes photochemically reactive organic gases such as xylenes, cycloolefins and other aromatics.
In the case of windrows and aerated static piles, the air that has passed through the compost mass is often allowed to dissipate into the atmosphere without treatment. In-vessel composters, however, use forced aeration systems that create significantly greater volumes of air being pushed through the compost mass. This continuous stream of exhaust gas must be treated to remove odorous and toxic compounds below human smell recognition threshold levels or toxic threshold levels before it is discharged into the atmosphere. Various treatment technologies are currently being used to treat the exhaust air, including: scrubbing with water, acids, caustics, and/or bleach; filtration through odor filter piles or other media (carbon); dilution with fresh air; and final discharge and dispersion through elevated stacks. Because the exhaust gas from the composter contains a complex mixture of compounds, more than one treatment method is often employed. Table 2 describes the odor control systems used by major composting facilities in the United States.
Despite the use of various odor control methods, existing in-vessel composting installations have been unable to insure consistent, adequate odor control or to adequately reduce the emission of toxic compounds. In addition, the aeration air is not recirculated into the composter which results in large and expensive exhaust air treatment facilities.